Electro-optic modulators (EOM) are commonly used in optical communication networks. A phase-modulating EOM may be used in a Mach-Zehnder interferometer to modulate the amplitude of an incoming optical signal. As is known, Mach-Zehnder based opto-electronic modulators have a relatively high power consumption, are large and require a high drive voltage.
Improving the bandwidth-density product in an integrated silicon photonic system requires a corresponding improvement in the performance of the optical modulator disposed in such a system. Optical modulation in conventional optical ring modulators is achieved either by varying the coupling level or by changing the index of refraction of the ring, either by injecting excess minority carriers in the associated PIN junction or by changing the reverse bias voltage applied to the PN junction. The change in the index of refraction causes a change in the optical path length of the ring, in turn changing the resonance frequency of the ring.
As is well known, conventional optical ring modulators are susceptible to temperature fluctuations. Furthermore, as the Quality factor (Q) of a conventional optical ring modulator increases to achieve lower power consumption and enhanced energy efficiency, the bandwidth of the optical ring modulator decreases. In other words, there is a trade-off between the optical bandwidth and power consumption of a conventional optical ring modulator.
FIG. 1A is a top schematic view of an optical ring modulator 100, as known in the prior art. Optical ring modulator 100 is shown as including, in part, a waveguide 20, and an optical ring 30. The optical signal entering waveguide 20 through its input port 10 is coupled to optical ring 30.
Optical ring 30 includes an intrinsic silicon region 36, a highly doped n+ region 32 encompassing silicon region 36, and a highly doped p+ region 34 formed within the inner area of silicon region 36. FIG. 1B is a cross-section view of optical ring 30 showing intrinsic silicon region 36, and the highly doped n+ and p+ regions 32 and 34, respectively. By changing the voltage applied between n+ region 32 and p+ region 34, the refractive index of the optical ring 30 is varied. The interference between the optical signals travelling through waveguide 20 and optical ring 30 modulates the optical signal at the output port 12 of optical ring modulator 100. Referring to FIG. 1C, signals 50 (delivered via input port 10) and 52 (delivered via output port 20) are exemplary input and output signals of the optical modulator shown in FIG. 1A.
The time-domain dynamic transmission of the ring modulator, T(t) may be defined as:
                              T          ⁡                      (            t            )                          =                              σ            ⁡                          (              t              )                                +                                                    k                ⁡                                  (                  t                  )                                                            k                ⁡                                  (                                      t                    -                    τ                                    )                                                      ⁢                          α              ⁡                              (                t                )                                      ⁢                                          exp                ⁡                                  [                                                            -                      i                                        ⁢                                                                                  ⁢                                          φ                      ⁡                                              (                        t                        )                                                                              ]                                            ⁡                              [                                                                            σ                      ⁡                                              (                                                  t                          -                          τ                                                )                                                              ⁢                                          T                      ⁡                                              (                                                  t                          -                          τ                                                )                                                                              -                  1                                ]                                                                        (        1        )            where σ and κ respectively represent transmission and coupling coefficients of the optical ring modulator, α represents the attenuation level, φ represents the phase shift inside the ring, and τ represents the travel time of the optical signal around the resonator, i.e., the round trip time in the resonator.
FIG. 2A shows the static transmission characteristic of a conventional optical ring modulator obtained through numerical solution of expression (1) using an iterative approach. FIG. 2B shows the Q-bandwidth response of such an optical ring modulator. Plots 60 and 62 of FIG. 2B respectively correspond to conventional optical modulators having Qs 3000 and 20000 respectively. It is seen that the modulator with a Q of 3000 has a wider bandwidth. FIG. 2C also shows the drop-off in the frequency response as the Q of the modulator increases. Referring concurrently to FIGS. 1C and 2C, it is seen that a conventional optical modulator, such as that shown in FIG. 1A, has a low-pass response.
FIG. 2D shows the static transmission characteristic of another exemplary optical ring modulator that receives an optical signal generated using a laser having a wavelength of nearly 1527.4 nm, as shown. Plots 70 and 72 respectively show the static transmission characteristic of the optical ring modulator for two different reverse-biased voltages. By changing the reverse-biased voltage of the P-N junction disposed in the ring, the index of refraction of the ring and hence the optical length of the ring changes, thereby resulting in a shift in the resonance frequency of the ring. In plot 70, the resonance frequency is shown to occur at wavelength of nearly 1527.2 nm associated with a reverse bias voltage of V1. In plot 72, the resonance frequency is shown to occur at wavelength of nearly 1527.3 nm associated with a reverse bias voltage of V2. The modulator's transmission value is seen to change from −9 dB (associated with an optical output of zero) to −3 dB (associated with an optical value of one) defining an extinction ratio of 6 dB. In other words, a change of ±(V2−V1) in the voltage applied to the reverse-biased p-n junction disposed in the modulator causes the optical output signal of the modulator to switch between one and zero.
FIG. 3A shows a schematic diagram of an optical modulator 150 that includes a variable coupler 160, as is also known in the prior art. FIG. 3B shows exemplary input/output signals of optical modulator 150. Output signal 175 supplied at output port 185 of optical ring modulator 200 is generated in response to optical input signal 170 delivered to input port 180 of the modulator. The modulation of optical signal 185 is achieved by changing the coupling ratio between optical path 190 of the modulator and optical path of 195 of the ring using variable coupler 160. As is seen from FIG. 3B, the relatively long sequence of input is at the input port of the optical modulator causes an energy droop in the ring and output signal degradation. Accordingly, optical modulator 150 may be characterized as having a highpass response. A need continues to exist for an improved optical ring modulator.